The purpose of this paper is to present the experience of Conservation International (CI) in working with partners in the Population Environment (PE) program and lessons learned from those experiences. The "Meeting Population and Conservation Needs in the Selva Lacandona" project partnerships are examined in this case study of CI’s PE program in Chiapas, Mexico, which was funded by the Davis and LucillePackard Foundation. Additional lessons learned from other types of partnerships in other PE programs are also included. Among these: maintaining a close, trusting relationship with partners helps to avoid a variety of problems. The more staff members have experience with partnership building, the easier it will become. However, if problems do occur, immediately evaluate the partnership and discuss steps to address the problems, by changing the partner’s scope of work, refining aspects of the work, or consider discontinuing the partnership.